Stimulation of oil and gas wells with phosphate ester surfactants

ABSTRACT

Crude oil and gas production wells, which no longer produce oil or gas utilizing conventional primary and secondary means of recovery, can be returned to production by treatment comprising addition of an aqueous solution of certain phosphate ester surfactants followed by successive treatment with a hydrocarbon. Increased production also can be obtained in low-producing oil and gas wells by similar treatment with certain phosphate ester surfactants. Subsequent to treatment of a producer well, the phosphate ester surfactant is forced into the formation utilizing a hydrocarbon and said surfactant and hydrocarbon mixture is allowed to remain in the producing well, for an effective period of time ranging from not less than 24 hours to one month. Thereafter, pumping and/or conventional fluid drive means are then utilized to recover oil or gas from the subterranean oil or gas formation. The amount of aqueous surfactant solution and hydrocarbon utilized are sufficent to permeate the oil or gas subterranian formation in the area immediately adjacent to the producing well bore and up to a radius therefrom of about 20 feet.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to oil and gas production well stimulation wheresaid wells are low-producing or have ceased to produce by primary andsecondary recovery means.

2. Description of the Prior Art

Certain phosphate esters have been used in conjunction with water-floodmethods of driving oil toward producing wells as a means of secondaryrecovery of hydrocarbons. The phosphate esters are added to an injectionwell and perform various functions such as scale inhibition, and theformation of a slug for driving the oil in the formation toward theproducing well. The phosphate ester can be injected into the wall eitheras an aqueous solution or as a soluble oil micro-emulsion. Alkyl andaralkyl polyoxyalkylene phosphates are disclosed as useful surfactantsin water-flood secondary recovery processes. Such processes aredisclosed in U.S. Pat. Nos. 3,435,898, 3,596,715, and 3,480,083.

Producing oil and gas wells have long been treated to stimulateproduction thereof utilizing a method termed "acidizing" in which anemulsion of an aqueous mineral acid either alone or in combination withvarious surfactants, corrosion inhibiting agents, and hydrocarbon oilsis added to a producer well. Presumably, such treatments tend to removedeposits from the area of the subterranean oil or gas formationimmediately adjacent to the production well bore, thus increasing thepermeability of the formation and allowing residual oil or gas to berecovered through the well bore. Another object of such "acidizing"treatment of oil or gas producer wells is the removal of water from theinterstices of the formation by the use of a composition whichmaterially lowers the interfacial forces between the water and the oilor gas. Various surface-active agents have been recommended for thisuse.

Usually, the surface-active agent is injected into the production wellin combination with crude oil or other hydrocarbon solvent followed bythe injection of additional crude oil to move the treating fluid intothe formation. U.S. Pat. Nos. 3,467,194 and 3,470,958 providespecifically for the treatment of a cude oil production well utilizingan oil-external micellar dispersion or a water-external micellardispersion followed by the injection of a hydrocarbon to move themicellar dispersion outwardly from the well bore into the formation.There is disclosed in U.S. Pat. No. 3,620,303 a method of treating aproduction well with an aqueous solution of an anionic orthophosphateester surfactant. The aqueous surfactant solution is retained in theformation at least about 24 hours and thereafter the solubilizedhydrocarbon is displaced toward the producer well by fluid drive meansutilizing a second well bore which is in fluid contact with theunderground oil formation. It is theorized that the surfactant solutionforms a soluble oil micro-emulsion with the formation hydrocarbons.

There is no indication in any of the prior art references that theparticular phosphate ester surfactants disclosed and claimed hereinwould be useful in returning to production oil and gas wells which arelow-producing or have ceased to produce. The method of the invention isparticularly useful in the stimulation of oil and gas wells which havefailed to respond to acidizing treatment of the producing well includingthe use of various acids with various surfactants.

SUMMARY OF THE INVENTION

There is disclosed a method of increasing production or returning toproduction oil and gas wells by the treatment of a production wellcomprising adding an aqueous solution of an oxyalkylated phosphate estersurfactant and thereafter adding a hydrocarbon to drive the aqueoussurfactant solution into the formation a distance of about 3 to about 20feet from the well bore. The aqueous surfactant solution is allowed toremain in the formation for an effective period generally not less than24 hours and preferably 24 hours to 1 month. Thereafter, the oil or gasis recovered using pump and/or conventional fluid drive means whereinwater is injected into an injection well in communication with said oilor gas subterranean formation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The oxyalkylated phosphate ester which is an active ingredient in theprocess disclosed herein for increasing production or returningnonproducing oil and gas wells to productivity is an alkyl or aralkylpolyoxyalkylene phosphate ester surfactant. The surfactant can be usedin the free-acid form or as the alkali metal or ammonium salt. Thephosphate esters of the invention have the formulas: ##STR1## wherein R₁represents an alkyl radical having 10 to 18 carbon atoms, for instance,nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl,hexadecyl, heptadecyl, octadecyl, nonadecyl, eicosyl, uneicosyl,docosyl, tricosyl, tetracosyl, pentacosyl, hexacosyl, and heptacosylradicals. R₂ represents an alkyl of about 5 to 27 carbon atoms, e.g.,pentyl, heptyl, hexyl, etc., higher alkyls of the same value as R₁,cycloalkyl, e.g., cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl,methylcyclopentyl, methylcyclohexyl, methylcyclooctyl, methylcyclobutyl,1,1-dimethylcyclopropyl, ethylcyclopropyl, ethylcyclobutyl,1-ethyl-2-methylcyclopropyl, 1,1,2-trimethylcyclopropyl,1,2,3-trimethylcyclopropyl, 1,1-dimethylcyclopentyl,1,2-dimethylcyclopentyl, 1,3-dimethylcyclopentyl,1,2-dimethylcyclohexyl, 1,3-dimethylcyclohexyl, 1,4-dimethylcyclohexyl,ethylcyclohexyl, ethylmethylcyclopentyl, 1,1,2-trimethylcyclopentyl,cyclononyl, propylcyclohexyl, 1,2,4-trimethylcyclohexyl,1,3,5-trimethylcyclohexyl, 1-isopropyl-4-methylcycohexyl,1,2,4,5-tetramethylcyclohexyl, and radicals derived from mineral oilscontaining alkyl, cycloalkyl and mixed alkylcycloalkyl radicals havingfrom about 12 to 27 carbon atoms. R₃ and R₄ represent either hydrogen,alkyl of from about 1 to 22 carbon atoms, e.g., methyl ethyl, propyl,butyl, and the higher alkyls defined by R₁ and cycloalkyls defined by R₂or radicals derived from mineral oils. A represents the residue ofethylene oxide, ethylene oxide and tetrahydrofuran, or mixed loweralkylene oxides selected from the group consisting of ethylene oxide,propylene oxide, and butylene oxide alone or including tetrahydrofuran,wherein the total molecular weight of said ester is about 500 to about1500, preferably about 600 to about 1200, and wherein A can be hetericor block in molecular configuration. The degree of oxyalkylation isrepresented by n, x and y are 1 or 2, the sum of x and y is 3, and z isan integer of 0 to 5; X is hydrogen or a monovalent cation selected fromthe group consisting of at least one of an alkali metal and ammonium. Inthe formation of the polyhydroxyl oxyalkylene polymer, at least onealkylene oxide or tetrahydrofuran is utilized. Preferably at least 2alkylene oxides or tetrahydrofuran and one alkylene oxide are utilized.In the formation of said phosphate ester surfactants, alkyl or aralkylalcohols can be reacted with alkylene oxides in accordance with wellknown prior art procedures.

The phosphorus acid reactants with which the polyhydroxy oxyalkylenecompounds are reacted in the formation of the phosphate esters of theinvention can be selected from the group consisting of at least one ofphosphorus pentoxide, and polyphosphoric acid. Where the polyhydroxyoxyalkylated compounds are derived from mixtures of lower alkyleneoxides or tetrahydrofuran, it is preferred that the compounds bemixtures of ethylene oxide and propylene oxide and that where block orheteric copolymers of these alkylene oxides are formed, it is preferredthat the proportion of ethylene oxide be about 10 to about 90 percent byweight and the proportion of propylene oxide be about 90 percent toabout 10 percent by weight. These surfactants can be utilized in theform of mono-, di- and mixed mono- and diphosphate esters. Where thesurfactants are utilized in the free acid form, the monoesters arecharacterized by the formulas above in which X is hydrogen.

The aqueous medium which is utilized to form the solution of thephosphate ester surfactant can be soft, brackish, or a brine.Preferably, the water is soft but it can contain small amounts of saltswhich are compatible with the ions in the subterranean oil or gasformation being treated.

The phosphate ester disclosed has been found superior to the surfactantsdisclosed in the prior art as useful in oil well stimulation and thusthe disclosed phosphate ester surfactant can be utilized as the solesurfactant in preparing the aqueous solution useful in treating the oilor gas production well. Generally, the concentration of said surfactantin the aqueous medium is about 5 to about 50 percent by weight,preferably about 10 to about 20 percent by weight, and most preferablyabout 12 to about 18 percent by weight. The amount of aqueous surfactantsolution utilized in the treatment of an oil or gas well will, ofcourse, vary with the vertical feet of oil-bearing formation around theproducing well bore. Generally, there is injected into the oil- orgas-bearing subterranean formation, about 0.5 to about 10 barrels of theaqueous surfactant solution per vertical foot of oil- or gas-bearingformation. There is thereafter added to the well sufficient hydrocarbonsuch as crude oil into the well bore to displace said aqueous surfactantsolution out into the formation. Generally, the aqueous surfactantsolution is retained in the formation not less than 24 hours andpreferably 24 hours to 1 month. Thereafter, the aqueous solution of saidphosphate ester and said hydrocarbon are displaced from the formation byconventional fluid drive means by use of an injection well in fluidcommunication with the treated production well. The desired oil or gasis thereby produced.

In addition to crude oil having generally the viscosity of theoil-bearing formation of the oil well to be treated, various hydrocarbonsolvents are also useful to displace the aqueous solution of surfactantout into the reservoir. Such hydrocarbon solvents as the low molecularweight, generally liquid hydrocarbons boiling below the gasoline range,such as the lower alkanes including butane, propane, pentane, hexane andheptane, as well as natural gasoline, petroleum naphtha and kerosene ormixtures of these hydrocarbons, are useful. Both sweet and sour crudeoil is useful as a hydrocarbon to displace the aqueous surfactantsolution out into the subterranean reservoir of oil or gas.

The following examples illustrate the various aspects of the inventionbut are not intended to limit its scope. When not otherwise specifiedthroughout this specification and claims, temperatures are given indegrees centigrade and parts, percentages, and proportions are byweight.

EXAMPLE 1

About 4 months after a Texas oil well having 17 feet of perforation, hadstopped producing oil, a 15 percent aqueous solution of a mixed mono-and diphosphate ester of a block copolymer of 85 percent by weightethylene oxide and 15 percent by weight propylene oxide initiated withan aliphatic alcohol having 10 to 12 carbon atoms and a molecular wightof about 900 was pumped into the well and displaced with approximately25 barrels of crude oil from the well. The well was allowed to standidle for about 72 hours after which it was found that a pressure head of60 pounds per square inch was built up. The pressure was insufficient tocause the well to flow so that pumping was started to recover oil. Thewell produced 37 barrels of oil per day for about 2 weeks. Productionthen declined to 25 barrels of oil per day where it remained for therest of the month. Previous to the treatment utilizing the process ofthe invention, this well had been acidized several times. Each acidtreatment failed to bring the desired results in restored production ofthe well.

EXAMPLE 2

A gas well in Southeast Texax, which had been producing 40,000 cubicfeet per day, was treated in accordance with the process of theinvention utilizing the phosphate ester of Example 1 in the amount of 1barrel per vertical foot of perforation. The aqueous phosphate estersolution was displaced from the well into the formation with waterrather than a hydrocarbon oil and the well was closed for a period of 72hours. At the end of this time, the well was opened up and productionoccurred at the rate of 1 million cubic feet of gas per day. Thisproduction rate was maintained for over 3 weeks.

While this invention has been described with reference to certainspecific embodiments, it will be recognized by those skilled in the artthat many variations are possible without departing from the scope andspirit of the invention and it will be understood that it is intended tocover all changes and modifications of the invention disclosed hereinfor the purpsoes of illustration which do not constitute departures fromthe spirit and scope of the invention.

The embodiments of the invention in which an exclusive privilege orproperty is claimed are defined as follows:
 1. A method of treating oil-and gas-bearing subterranean formations to improve permeability thereofand increase or restore production of oil or gas comprising introducinginto said formation through a producing means in fluid communicationwith said oil- or gas-bearing subterranean formation, a treating fluidcomprising:(A) an aqueous solution of about 5 to about 50 weight percentof an alkyl or aralkyl polyoxyalkylene phosphate ester surfactant havingthe formulas: ##STR2## wherein R₁ represents an alkyl radical containingfrom 10 to 18 carbon atoms, R₂ represents an alkyl radical of about 5 toabout 27 carbon atoms or a cycloalkyl radical and radicals derived frommineral oils containing alkyl, cycloalkyl and mixed alkylcycloalkylradicals having from about 12 to 27 carbon atoms, R₃ and R₄ representeither hydrogen or alkyl of from about 1 to 22 carbon atoms and thehigher alkyls defined by R₁ and cycloalkyls defined by R₂ or radicalsderived from mineral oils; A represents the residue of ethylene oxide,ethylene oxide and tetrahydrofuran, or mixed lower alkylene oxidesselected from the group consisting of ethylene oxide, propylene oxide,butylene oxide, alone or including tetrahydrofuran, wherein the totalmolecular weight of said ester is about 500 to about 1500, and wherein Acan be heteric or block in molecular configuration; n represents thedegree of oxyalkylation; x and y are 1 or 2, the sum of x and y is 3 andz is an integer of 0 to 5; X is hydrogen or a monovalent cation selectedfrom at least one of the group consisting of an alkali metal, andammonium; and thereafter, (B) injecting a hydrocarbon into the porespace adjacent to the well bore in amounts sufficient to displace theaqueous solution of said phosphate ester out into the formation, (C)retaining said aqueous solution of said phosphate ester and saidhydrocarbon within said formation for a period of not less than 24 hoursand (D) thereafter, pumping and/or displacing said aqueous solution ofsaid phosphate ester and said hydrocarbon from said formation into thewell bore by injecting water into said formation by an injection meansin fluid communication with said oil-bearing or gas-bearing subterraneanformation to produce the desired oil or gas through said producingmeans.
 2. The process of claim 1 wherein said aqueous solution of saidphosphate ester surfactant is injected into said formation at the rateof about 0.5 to about 10 barrels of said aqueous solution per verticalfoot of oil- or gas-bearing formation and retained in the subterraneanoil or gas formation for a period of 24 hours to 1 month.
 3. The processof claim 2 wherein said hydrocarbon is produced oil or a hydrocarbonboiling below the gasoline range.
 4. The process of claim 3 wherein saidproducing well is an oil producing well, said phosphate ester is (I),prepared by reacting a polyhydroxy oxyalkylated compound with aphosphorus acid reactant selected from the group consisting ofphosphorus pentoxide, polyphosphoric acid, and mixtures thereof.
 5. Theprocess of claim 4 wherein said polyhydroxy oxyalkylated compound is ablock or heteric copolymer wherein A represents the residue of ethyleneoxide, ethylene oxide and tetrahydrofuran, or mixed lower alkyleneoxides selected from the group consisting of ethylene oxide, propyleneoxide and butylene oxide alone or including tetrahydrofuran.
 6. Theprocess of claim 5 wherein said polyhydroxy oxyalkylene compound is thereaction product of at least one monohydric alcohol having 10 to 15carbon atoms.
 7. The process of claim 6 wherein said polyhydroxyoxyalkylated compound is a block copolymer of ethylene oxide andpropylene oxide having a molecular weight of about 600 to about
 1200. 8.The process of claim 7 wherein said block copolymer is prepared bysuccessively reacting ethylene oxide and propylene oxide in theproportion of 10 to 90 percent by weight to 90 to 10 percent by weightrespectively.
 9. The process of claim 8 wherein said block polymer isprepared by successively reacting 15 percent by weight propylene oxidefollowed by 85 percent by weight ethylene oxide with a monohydricaliphatic alcohol having 10 to 12 carbon atoms.
 10. The process of claim9 wherein said block polymer has a molecular weight of 900.